This invention relates to aquarium filters, and more particularly to an external aquarium filter providing biological, mechanical and chemical filtration which can be positioned under the aquarium tank.
It is generally understood that the proper maintenance of an aquarium requires continuous water filtration, preferably of three types as follows: (1) mechanical filtration, which removes suspended solid material from the water, (2) chemical filtration, which removes dyes and other colorants as well as certain inorganic substances, and (3) biological filtration, which removes certain organic waste products by means of naturally occurring bacteria living attached to the filter material. It is clear that biological filtration is most effective if a large surface area for bacteria attachment is provided in the filter.
Typical aquarium filtration devices include a housing containing filter material through which the contaminated aquarium water passes into a clean water chamber. A siphon is generally utilized for extracting the contaminated water from the aquarium tank and supplying it to the filter. The clean water is then either pumped or airlifted back into the aquarium tank thereby providing a continuous flow of water through the aquarium filtration device and providing both clean water to the aquarium tank as well as providing the necessary aeration needed into the tank.
Such external aquarium filters are usually hung over the edge of the aquarium tank and depend therefrom.
The type of filter material utilized in the external aquarium filters generally includes a layer of charcoal with an additional layer of glass wool or polyester fibers placed above the charcoal.
While these filters generally provide sufficient mechanical and chemical filtration, they do not provide adequate biological filtration. The surfce area of the filter material is much too small to support the large bacteria population needed to successfully convert dissolved organic waste products into harmless substances. Furthermore, these filters clog rapidly and need to be changed regularly. Since all filter material is discarded at each change, the small bacteria population which had become established and would have removed at least some of the accumulating dissolved waste products, is discarded as well.
By way of example, two such external aquarium filters are described in my U.S. Pat. Nos. 3,312,082 and 3,554,375.
Although the use of the polyurethane foam as a filter material in the external aquarium filter has been suggested, this type of device still has numerous problems. For example, as the contaminated water flows downwardly through the filter material, the debris will settle over the filter material and cause clogging. This will especially become a problem when utilizing the polyurethane foam. The larger particles of detritus and food flowing with the contaminated water will clog the pores of the foam to completely impair the functioning of the filter. Upon necessary thorough cleaning thereof, the whole bacteria population will be lost.
Another type of aquarium filter is the undergravel type of filter. This device includes a perforated tray spaced from the bottom of the aquarium tank on which is suspended a layer of gravel about 1/2" above the bottom of the aquarium floor. The contaminated aquarium water passes through the gravel, which serves as the filter bed, and proceeds through the perforations in the tray. The filtered water is then airlifted back into the aquarium proper.
Within the gravel, the water is filtered mechanically. However, the efficiency of this process depends upon the size of the gravel grains, the depth of the filter bed, and the flow rate of the water. In addition to the mechanical filtering actions, the undergravel filter serves as a biological filter. Certain species of aerobic bacteria, which grow attached to the gravel grains and to particles of detritus, convert potentially toxic by-products of fish metabolism into non-harmful products.
For example, ammonia and nitrite, which are potentially toxic by-products of fish metabolism, are converted into nitrate and nitrogen. These latter chemicals which even in high concentrations are non-toxic to fish, can be used either by the plants, as in the case of nitrate, or else can escape through the surface of the water directly into the air, as in the case of nitrogen.
Numerous types of such undergravel filter devices are readily available. For example, U.S. Pat. No. 2,935,199, issued to the present inventor, describes one such undergravel filter. Numerous other types are clearly available.
The undergravel filters have been well accepted by hobbyists and, if properly installed and maintained, serve a very important function in the aquarium. However, there are a number of drawbacks to these filters which require the hobbyist to adhere to strict precautionary measures. They also place restrictions upon the use of decorative materials and plants in the aquarium.
For example, for greatest efficiency of flow, the diameter of the gravel grains should be of a size from about 2 to 5 mm. The particles should also be irregularly shaped, without being smooth and round, in order to achieve the greatest possible surface area for filtration. However, this type of gravel is not always available to the user, or it might not fit his decorative scheme.
In order to achieve an even flow of the water through the gravel filter bed, the gravel should be placed in an even uniform layer. Any mound building should be avoided since the water will find the least resistant passage through the gravel and will therefore not flow through the heaped up gravel portions. At the same time, the optimal thickness of the filter bed should be about 2 to 3 inches. Either more or less amount of gravel decreases proper functioning of the filter bed.
Additionally, the contents of the aquarium tank itself often disturb the gravel filter bed. For example, certain types of fish can dig depressions into the bed, often down to the filter plate, thereby rendering the filtration system useless. Additionally, it has been found that aquatic plants do not flourish in aquariums using undergravel filters. Although the reason for this is not quite clear, it may be that a biological imbalance occurs within the gravel bed, or that the water flowing past the roots disturbs the growth of the plants. Furthermore, rocks or other decorations placed on the gravel bed will interfer with the even water flow causing dead spots in the filter bed. Similarly, terracing of the gravel bed destroys the uniformity of depth thereby decreasing the filtering action.
The underground filters maintained within the aquarium tank also provide additional problems. Oftentimes, especially with beginning hobbyists, the fish will be overfed. This causes the uneaten food particles to clog the interstices between the gravel grains, thereby overloading the capability of the filter bed to convert organic matter. Removal of the accumulated food particles as well as accumulations of detritus to unclog a filter bed requires additional filtering apparatus.
Such accumulations occur not only in the case of food particles, but even during continuous filtration. Since the water flows down through the gravel, heavy sediment collects on top of the gravel layer which causes the filter bed to become partially clogged. Channels will form in places of least resistance in the filter bed, for example, where the coarser grains provide large interstices. This will drastically reduce the effective filtering area. Ultimately, the filter bed will become completely clogged, necessitating a thorough washing of the gravel, or even a complete breakdown of the aquarium.
In using the undergravel filter, the airlift brings the aerated water within the aquarium tank. Accordingly, it is very difficult to determine the flow rate through the airlift tube especially when the tubes are at least partially submerged in water. Although air bubbles may be seen rising in the tube, water may not be flowing if the gravel bed is clogged. The air stream continues to work but the water may not be circulating. Since it may go unnoticed that filtration has ceased in the aquarium, a dangerous buildup of toxic waste products may occur.
In using the undergravel filter, there is no practical means of utilizing activated carbon for providing chemical filtration in the system without seriously impairing the flow.
Perhaps the most critical problem in connection with undergravel filters is that they require the use of gravel in the aquarium tank since the gravel forms the filter bed. Breeders of aquarium fishes and wholesale establishments often use no gravel at all. As a result, they cannot make efficient use of this type of biological filtration.
In order to achieve the effect of a biological filter, without utilizing the undergravel type filter, it has been suggested to utilize a polyurethane sponge instead of gravel on the bottom of the aquarium. This approach has been suggested by Art Hayley, in an article, "The Biological Sponge" published in Freshwater and Marine Aquarium Magazine, January, 1978, Vol. 1, Number 1, pages 33-37 and 81.
While the sponge would harbor a sizable population of bacteria, the problems of clogging and consequent cleaning without disturbing the biological filtering capacity still exist.
In summary, although both the conventional external filter and the undergravel filter perform important functions in the aquarium, neither can combine mechanical, chemical and biological filtration into one filter.